Diaphragm unit and method of fabricating same



P. H. BILHUBER 2,529,862

DIAPHRAGM UNIT AND METHOD OF FABRICATING SAME Nov. 14, 1950 Filed June22, 1943 Patented Nov. 14, 1950 UNITED STATES PATENT orrics DIAPHRAGMUNIT AND METHOD OF FABRICATING SAME Paul H. Bilhuber, Douglas Manor, Y.,assignor to Steinway & Sons, New York, N. Y., a corporation of New YorkApplication June22, 1943, SerialNo. 491,782

1 Claim. 1 This invention relates to vibrating diaphragm units formusical instruments such as piano soundboards, violin sound boxes, reedsof woodwind instruments and the like and relates more particularly tocertain new and usefulimprovements in vibrating diaphragm units of thenature aforesaid and in'methods of fabricating such units.

Objects and advantages of the invention will fin embodiment of thisinvention and apparatus for carrying out the process of the inventionand together with thedescription serves to explain the principles of theinvention ,Objects of this invention are to provide an improveddiaphragm unit for pianos or other ,wood string or wood wind musicalinstruments and to provide an improved method of fabricating such units;to provide an improved method of conditioning wood, particularly wood tobe fabricated into diaphragm'uni'ts for musical instruments, to renderthe same less hygroscopic to provide a piano soundboard which is a morehomogeneous structure, which is less hygroscopic and hence will be lesssubject'to cracking, shrinkingj warping, buckling, swelling and suchform of deterioration with changes in atmospheric conditions whch impairthe quick speaking properties of a soundboard; which will have a lowerdensity for a given modulus of elasticity and strength; which will haveless inertia and so possess an improved tone and enable the instrumentto speak more quickly; and, to provide a new and improved method formanufacturing piano Soundboards and like diaphragm units which will"effect material economies in materials, in manufacturing cost, inmanufacturing time and equip- I'nent and through the use of whichsoundboards having a more favorable ratio of elasticity to density canbe produced from a given material F board constituting a typical andillustrative embodiment of this invention;

plied to the fabrication of a piano soundboard,

Figure 2 is a view in section taken along "the line 2-2 of Figure 1showing the soundboard with its bridge and ribs, applied to the frame ofa piano; and,

Figure 3 is a diagrammatic view of o'ne' form of high frequencyelectrostatic apparatus for use in the carrying out of the method ofthisinvenjtion. 2

In accordance with the invention, the wood from which the diaphragm unitis to be fashioned or fabricated, is'conditioned by subjecting the woodto a short time, high temperature electrostatic baking operation. Theoperation serves to reduce the mass (density) of the wood appreciably,to render the wood less hygroscopic thereby substantially preventingsubsequent swelling, cracking, checking or warping of the wood andenables the accomplishment of these results without reducing the modulusof elasticity of the wood, or its strength (except for possibly a'reduction in impact strength) and "without objectionable discolorationcrease-hardening of the material.

The wood to be processed is preferably in the form of strips or elementsfrom one or more of which the diaphragm unit such as a piano soundboard,a violin box, or a reed of a wood wind instrument is subsequently to becut, fashioned,

shaped or assembled, as the case maybe.

In accordance with the invention and; as apwood strips of alengt-h,width and thickness preferably in the range 2' to 9', 3'? to 8 and /8 to1%, respectively, are placed between, andj'pref- 'erably in contactwith, the electrodes of high "frequency electrostatic heating apparatussuch, for :examplaas is shown in "Figure ,3.

r The strips ;may be stacked or piled one upon the other, but preferablyare arranged insideby-side relation so as to provide a single thicknessried to suit the capacity of the apparatus dependin rupon the stripdimensions.

The power supplied tothe electrodes should, be sufficient, having regardto the dielectric constant (e. g. 215 to"7.7) of the wood as a measureof its capacity to absorb power and be heated electrosta'tically, toestablish within a practically use- 'ful time a temperaturesubstantially uniformly internally of the wood, at which in addition tofree moisture such constituents as oleo resins, e.

g; rosin, pine oil, terpenes; acids, e.- g. formic acid and acetic acid;and, particularly wat'er-of-con stitution of the. cellulose, will bedriven off.

A temperature in the range approximately 375 F. to approximately 425 F.preferably about 400 F., will, for most woods, be found suitable foreffectingthe removal of an optimum quantity of the constituents of thenature aforesaid. An ultra high frequency electrostatic field of aboutsix to eight megaoycles at an inner-electrode potential of about 5000 to6000 volts per inch of spacing will in general be found suitable for theobtainment of the desired temperature. At this temperature, the quantityof constituents driven off in the short time required, for example, to30 minutes depending upon the wood, including its free moisture content,and the rate of energy input, will be such as to achieve a reduction indensity appreciably greater, that is to say, greater by from about 4% to10%, than is possible of obtainment by the customary relatively slowkiln drying operations (3 to 14 days) conventionally employed in thepreparation of wood for-use in soundboards and normally serving to driveoff only free moisture. 1

Through the application of the electrostatic baking; operation with its,relatively short time, internal heating of the Wood, excessive surfaceheating -is avoided and discoloration due to scorching, which generallyaccompanies convective heat baking, is prevented. In consequence, thereis avoided the planing ofthe wood surface required to remove scorchedareas usually caused by convective baking which planing is ordinarilyfollowed by painting of the planed surface as with aluminum paint, tocover discolored areas. The added mass of the aluminum coatsubstantially nullifies mass-reduction achieved'by convective baking. Inaddition, radial cracking from tangential stresses induced by therelatively rapid surfacedrying and slower internal dryingcharacterizingv the usual kiln drying operations is avoided, so thatstock loss is minimized with concomitant manufacturing economies-. Thecombination of air drying and the conditioning of .the wood inaccordance with this invention minimizesthe stock losses consequent uponnormal kiln drying .operations; Moreover, wood which has been processedinaccordance with this invention possesses the definite advantage thatreabs or ption of moisture is held to a low percent age becausethe woodis less hydroscopic than by continuously circulating therein a coolingliquid .to. maintain them at a relatively low temperatureof, forexample, 32 F. Or, the baking operation may be-made intermittent innature by interrupting the, power input for short intervals so as toestablish a hot interior and a relatively cooler exterior .of the wood,a suitable temperature differential being, for example, in the order of15 to 30 F., to prevent scorching. For, example, the WOOd'IIlEtY beheated for 5 to '15 minutes, the current interrupted for 2 to 10minutes, with ,or without cooling of the electrodes,

and heating resumed for another 5 to -15 minutes. The conditioned woodstrips after removal from the-electrostatic baking apparatus, arecarefully graded-so as to select therefrom those having substantiallysimilar. physical properties particularly as regards equivalency ofdensity and modulus of elasticity. These strips are assembled inside-by-side relationship and adhesively bonded together in accordancewith practices well known in the art, to provide a soundboard blank ofsuitable dimensions from which the piano soundboard is cut and fashionedin a manner known to those skilled in the art.

The soundboard may be sized with a suitable lacquer followed byvarnishing, preferably with an elastic varnish, thus sealing the woodagainst moisture absorption and at the same time improving itsappearance.

Referring now more particularly to the accompanying drawings, there isdepicted in Figure 1 a crown shaped piano soundboard I of the typedisclosed in United States Patent No. 2,070,391 granted February 9,1937, having its greatest thickness at 2. The thickness decreases towardits rim portion and is smallest at the rim portion indicated by thecurved dot-dash line 3. Intermediate contour lines 4, 5 and 6 servegeographically to depict the crown shape of the soundboard whosegreatest thickness, at the point 2, is at the geographical center of thesoundboard.

Bridges I and 8 are secured to the upper convex. preferably parabolicsurface 9 of the soundboard with the larger bridge 'Iof the twoprefferably passing .over substantially the geographic center.

The lower concave surface I0 of the soundboard has secured thereto ribsI I which together with the bridges I and 8 serve to hold the-soundboardin its crown shape with the fibres of the upper convex surface 9, intension and of the lower concave surface I0 in compression.

The soundboard is supported as shown in a piano frame I2 on the shoulderI3, a shim I4 being provided betweenthe soundboard and shoulder betweenthe points X and Y of Figure 1 so as to furnish equalized support withthe remaining parts of shoulder I3 of the pianoframe, not covered byshim I4.. I

The soundboard I; is cut from a blank'I5, depicted in outline forminFigure 1, comprising an assembly of grated strips I6 each carefullyselected from stock electrostaticallybaked in the manner previouslydescribed so as. to have sub,- stantialequivalency of physicalproperties, particularly as regards density. and .modulus of elasticity,and assembled in edge-joined relationship. .Because of-thispriorconditioning and careful selection, the soundboard I is a morehomogeneousl structure and. hence provides quicker response in itsdiaphragm action in use so that the instrument .speaks more quickly. Thesoundboard is highly resistant .to swelling,

cracking, warping, checking and such forms of deterioration fromclimatic conditions.

.In Figure 3 there is depicted indiagrammatic fashion a form ofhigh-frequency electrostatic apparatus for use in carrying out themethod of this invention. As there shown,-a press I! has its top andbottom relatively movable platens I8 and 59 respectively connected asone electrode. to

one terminal of an electrical high frequency generator-e. Ihe otherterminal of the, generator is connected to a plate 20 forming the otherelectrode. v Wood strips are positioned between the plate 20 and thepress platens in upper and lower layers 2E and 22 respectively, to besubjected to the high frequency field generated by the generator e inthe manner and for the purpose described. The electrodes l8, I9, and-20may be hollow and provided with nozzles (not shown) for circulatingcooling liquid in the electrodes.

It will be understood that the discoloration or scorching which woodsundergo in convection baking operations is more pronounced with somewoods than with others. Spruce of the Sitka, Engelmann, Rumanian, Rubra,White or Black variety preferred for piano soundboards discolors mosteasily, and hence may be operated on with advantage in accordance withthis invention. Other conifers desirably employed for soundboards, suchfor example, as Noble Fir, Western Hemlock, Redwood and Cypress are lessdifficult in this respect but are advantageously operated on inaccordance with this invention. Pine whether of the White, Ponderosa orSugar variety also discolors easily and may be operated on with similaradvantages as may Yellow Poplar although the latter is not subject totoo ready discoloration.

The invention in its broader aspects is not limited to the specificsteps, processes, combinations articles and improvements shown anddescribed but departures may be made therefrom within the scope of theaccompanying claims without departing from the principles of theinvention and without sacrificing its chief advantages.

What I claim is: The method of conditioning wood to be fabricated into adiaphragm unit for a musical instrument which compriseselectrostatically baking the wood in an ultra high-frequencyelectrostatic field of about six to eight megacycles for a time and at atemperature in the range approximately 375 F. to approximately 425 F. soas to remove oleo resins and at least some of the water of constitutionfrom the wood while cooling at least a portion of the wood surface so asto maintain a termperature difierential in the order of 15 to 30 F.between the inside and outside of the wood and thereby avoiddiscoloration of its surface.

PAUL H. BILHUBER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

